Clock distribution chip

ABSTRACT

In one embodiment, a clock distribution chip includes a first clock input adapted to receive a first single-ended input clock signal, a second clock input adapted to receive a second single-ended input clock signal, and input buffer circuitry coupled to the first and second clock inputs. The input buffer circuitry is adapted to select an input clock signal among the first single-ended input clock signal, the second single-ended input clock signal, and a differential input clock signal derived from the first and second single-ended input clock signals. A phase-locked loop (PLL) is adapted to receive an input clock signal selected by the input buffer circuitry and to generate a PLL clock signal based on the selected input clock signal. A clock output provides an output clock signal based on the PLL clock signal.

RELATED APPLICATION DATA

This application is a continuation of U.S. application Ser. No. 11/425,881, filed Jun. 22, 2006, which is incorporated by reference herein in its entirety.

TECHNICAL FIELD

The present invention relates to integrated circuits, and, in particular, to clock distribution chips designed to distribute clock signals to devices on a circuit board.

BACKGROUND

A clock distribution chip (also referred to as a clock chip) is an integrated circuit (IC) designed to distribute clock signals to other electronic devices, such as other devices located on the same printed circuit board (PCB) as the clock distribution chip. Traditionally, there have been two different types of clock distribution chips: (1) zero-delay buffers and (2) non-zero-delay buffers (also referred to as simple clock buffers or drivers).

A zero-delay buffer receives an input (i.e., reference) clock signal and generates multiple output clock signals having specified (e.g., zero) phase offsets relative to the input clock signal. Zero-delay buffers typically employ phase-locked loops (PLLs) to generate the output clock signals, where the frequency of each output clock signal may differ from (e.g., may be greater or smaller than) the frequency of the reference clock signal. Some PLL-based zero-delay buffers offer skew control to select desired, non-zero phase offsets for one or more of the output clock signals relative to the reference clock signal.

Non-zero-delay buffers provide multiple copies of an input clock signal without ensuring that the generated output clock signals have any predictable phase offsets relative to the input clock signal.

Non-zero-delay buffers are typically non-PLL-based circuits.

In order for different devices, e.g., located on a single PCB, to be able to communicate, each transmitting device must transmit signals that each corresponding receiving device will recognize. Signaling standards have been promulgated to specify the characteristics of signals to enable such inter-device communications. Some signaling standards, such as LVPECL and LVDS, involve differential clock signals, while other signaling standards, such as LVTTL, LVCMOS, HSTL, and SSTL, involve single-ended (SE) clock signals. In addition, different signaling standards involve different voltage amplitudes, including some individual signaling standards having different versions at different voltage amplitudes. For example, there are three different types of LVCMOS signaling that are based on 1.8V, 2.5V, and 3.3V amplitudes, respectively.

Conventional zero-delay buffers and conventional non-zero-delay buffers come in particular models, each of which receives a limited variety of input clock signals and generates a limited variety of output clock signals (e.g., different models for different voltage amplitudes). For many applications, PCBs have different devices that require different clock signals, e.g., conforming to different signaling standards, for their processing, where some devices may require zero-delay clock signals, while other devices might not be able to tolerate the jitter typically inherent in PLL-generated zero-delay clock signals. As a result, a PCB for such an application is typically configured with a relatively large number of different models of clock distribution chips in order to support the variety of different signaling standards required by the devices on that PCB, resulting in possible wastage of resources.

SUMMARY

In one embodiment of the invention, a clock distribution chip includes a first clock input adapted to receive a first single-ended input clock signal, a second clock input adapted to receive a second single-ended input clock signal, and input buffer circuitry coupled to the first and second clock inputs. The input buffer circuitry is adapted to select an input clock signal among the first single-ended input clock signal, the second single-ended input clock signal, and a differential input clock signal derived from the first and second single-ended input clock signals. A phase-locked loop (PLL) is adapted to receive an input clock signal selected by the input buffer circuitry and to generate a PLL clock signal based on the selected input clock signal. A clock output provides an output clock signal based on the PLL clock signal.

In another embodiment of the invention, a clock distribution chip includes a clock input adapted to receive an input clock signal, a phase-locked loop (PLL) adapted to generate a PLL clock signal based on the input clock signal and a feedback clock signal, and a at least one programmable clock output for providing an output clock signal based on the PLL clock signal. The clock output is configurable to support a plurality of signaling standards. A feedback input is adapted to receive an output clock signal from the clock output. Programmable feedback buffer circuitry is coupled between the feedback input and the PLL. The feedback buffer circuitry is adapted to derive a feedback clock signal from the output clock signal received at the feedback input and to provide the feedback clock signal to the PLL.

These and additional embodiments of the invention are described and shown in the following detailed description and figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, features, and advantages of the present invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings in which like reference numerals identify similar or identical elements.

FIG. 1 shows a simplified block diagram of a clock distribution (CD) chip according to one embodiment of the present invention; and

FIG. 2 shows a more detailed block diagram of the input buffer circuitry of the CD chip of FIG. 1.

DETAILED DESCRIPTION

Chip-Level Description

FIG. 1 shows a simplified block diagram of a clock distribution (CD) chip 100, according to one embodiment of the present invention. Depending on its configuration, CD chip 100 receives either (1) one or two single-ended (SE) input clock signals (REFA and/or REFB) at input pins 102 and/or 104, respectively, or (2) the two halves (REFP and REFN) of a differential input clock signal at input pins 102 and 104, respectively, and generates up to 20 different SE output clock signals BANK_i[A/B] presented at 20 different output pins on CD chip 100.

In CD chip 100, the output pins are arranged in 10 different banks (BANK_0 to BANK_9), where each bank (i.e., BANK_i) has four pins: two output-clock pins (corresponding to output clock signals BANK_i[A] and BANK_i[B]), a power-level pin (corresponding to output power level

VCCO_i), and a ground-level pin (corresponding to output ground level GNDO_i), where VCCO_i and GNDO_i specify the voltage amplitude and ground levels, respectively, for the two corresponding output clock signals.

CD chip 100 can be configured to receive a relatively wide variety of different input clock signals and generate a relatively wide variety of different output clock signals. Table I shows the different types of input clock signals supported by CD chip 100, while Table II shows the different types of output clock signals supported by CD chip 100. Note that, for SE input clock signals, REFA and REFB can correspond to any combination of two SE signaling standards listed in Table I.

TABLE I INPUT CLOCK SIGNALS Standard Voltage Amplitude LVTTL 3.3 V LVCMOS-3.3 3.3 V LVCMOS-2.5 2.5 V HSTL 1.5 V or 1.8 V SSTL2 2.5 V SSTL3 3.3 V LVPECL 3.3 V LVDS 2.5 V or 3.3 V

TABLE II OUTPUT/FEEDBACK CLOCK SIGNALS Standard Voltage Amplitude (VCCO) LVTTL 3.3 V LVCMOS-3.3 3.3 V LVCMOS-2.5 2.5 V HSTL 1.5 V or 1.8 V SSTL2 2.5 V SSTL3 3.3 V

The upper left-hand corner of FIG. 1 shows a simplified version of the input buffer circuitry used to process the signals received at pins 102 and 104. FIG. 2, which is described later in this specification, provides a more complete diagram of this input buffer circuitry. Nevertheless, the circuitry shown in FIG. 1 accurately represents at least some of the functionality provided by CD chip 100. In particular, FIG. 1 shows a differential input buffer (e.g., op-amp) 106, two SE input buffers 108 and 110, and three (2×1) muxes 112, 114, and 116.

Differential input buffer 106 receives the two signals applied to pins 102 and 104 (to handle differential input clock signals REFP and REFN), while SE input buffers 108 and 110 receive the two signals applied to pins 102 and 104, respectively (to handle SE input clock signals REFA and REFB, respectively). The outputs from input buffers 106 and 108 are applied to the two inputs of mux 112, while the outputs from input buffers 106 and 110 are applied to the two inputs of mux 114. The outputs from muxes 112 and 114 are applied to the two inputs of mux 116. If CD chip 100 is configured to receive a differential input clock signal REFP/REFN, then at least one, if not both, of muxes 112 and 114 are controlled to select the output from differential input buffer 106, and mux 116 is controlled to apply that same clock signal as reference clock signal 118 to PLL core 120.

If CD chip 100 is configured to receive one or two SE input clock signals REFA and/or REFB, then muxes 112 and/or 114 are correspondingly controlled to select the outputs from SE input buffers 108 and 110, respectively, and mux 116 is controlled to select one of the outputs from muxes 112 and 114 to apply to PLL core 120 as reference clock signal 118.

As shown in FIG. 1, the selection made by mux 116 is based on an externally generated control signal REFSEL applied to pin 122.

As also shown in FIG. 1, in addition to being applied to muxes 112 and 114, the outputs from buffers 108 and 110 are also applied as clock signals 124 and 126 directly to (5×20) switch fabric 128, thereby bypassing PLL core 120 and dividers 130, 132, and 134. In an alternative embodiment, the outputs from muxes 112 and 114 could instead be applied as clock signals 124 and 126, respectively.

PLL core 120 comprises phase detector (PD) 136, loop filter 138, voltage-controlled oscillator (VCO) 140, clock divider 142, and (2×1) mux 144. PD 136 receives reference clock signal 118 from mux 116 as the PLL reference clock signal and feedback clock signal 146 from feedback buffer circuitry 148 as the PLL feedback clock signal. Feedback buffer circuitry 148 receives clock signal FBK applied to pin 150. In a preferred configuration, pin 150 is externally connected to receive one of the output clock signals BANK_i[A/B] generated by CD chip 100.

The output from PD 136 is filtered by loop filter 138, and the resulting voltage is applied to

VCO 140, which generates a corresponding VCO clock signal 152. Clock divider 142 divides VCO clock signal 152 by a divisor value of 2 and applies the resulting divided VCO clock signal 154 to one input of mux 144, which also receives a copy of reference clock signal 118 from mux 116 as its other input. Based on an externally generated control signal PLL-BYPASS applied to pin 155, mux 144 selects either reference clock signal 118 (thereby bypassing the PLL) or the PLL-based divided VCO clock signal 154 as output clock signal 156 from PLL core 120.

In either case, PLL core output clock signal 156 is applied in parallel to three different clock dividers 130, 132, and 134, each of which divides the PLL core output clock signal by a specified (and potentially different) divisor value to generate a corresponding divider output clock signal (158, 160, 162). In one implementation, each clock divider is a 5-bit divider capable of dividing the received clock signal by any integer divisor value from 1 to 32.

(5×20) switch fabric 128 (a) receives up to five different clock signals: (i) clock signals 158, 160, and 162 from clock dividers 130, 132, and 134 and (ii) clock signals 124 and 126 from the input buffer circuitry and (b) outputs up to 20 different output clock signals, in a fully non-blocking manner, such that each received clock signal can be routed to any one or more different clock signals 164.

Each clock signal 164 from switch fabric 128 is applied to a different skew controller 166, which adds a specified phase delay to the corresponding clock signal 164 based on a skew control signal 168 received from skew manager 170. In one implementation of CD chip 100, skew control is applied only to those clocks 164 derived from the PLL's VCO clock signal 152. In this implementation, each skew controller 166 has a (2×1) output mux (not shown) that receives both the non-skewed clock signal 164 received from switch fabric 128 and the skewed clock signal, where the mux output signal is selected based on whether or not the clock signal received from switch fabric 128 was derived from the PLL's VCO clock signal.

Skew control signals 168 generated by skew manager 170 instruct the relevant skew controllers 166 to add a phase delay corresponding to anywhere from 0 to 15 skew steps, where skew manager 170 determines the skew step size based on the PLL's VCO clock signal. In one implementation, skew manager 170 can be configured to operate in either a fine skew mode (where the skew step size is based on the VCO clock signal divided by 8) or a coarse skew mode (where the skew step size is based on the VCO clock signal divided by 4). In one implementation, each different skew controller 166 receives its own skew control signal 168 from skew manager 170. As such, different skew controllers 166 can be instructed to apply different amounts of skew to their respective clock signals.

The output of each skew controller 166 is applied to a programmable output drive buffer 172, which can selectively control the slew rate of the resulting output clock signal BANK_i[A/B]. Although not shown in FIG. 1, programmable on-chip termination circuitry is also provided to enable each output clock signal BANK_i[A/B] to have a variety of different termination levels corresponding to the different signaling standards specified in Table II.

As shown in FIG. 1, CD chip 100 has in-system programmable (ISP) Joint Test Action Group JTAG interface 174, which allows CD chip 100 to be programmed using a serial protocol and allows access to every pin of CD chip 100 to verify proper connectivity with the circuit board.

CD chip 100 may be said to include:

-   -   Input buffer circuitry comprising differential input buffer 106,         first-input input buffer 108, second-input input buffer 110, and         muxes 112-116;     -   Clock generation and distribution circuitry comprising PLL core         120, clock dividers 130-134, and switch fabric 128;     -   Fanout circuitry comprising skew controllers 166 and output         drive buffers 172; and     -   Feedback buffer circuitry 148.         Input Circuitry

FIG. 2 shows a more detailed block diagram of the input buffer circuitry of CD chip 100 of FIG. 1. Comparing FIG. 1 and FIG. 2, SE input buffer 108 of FIG. 1 is actually implemented using three different input buffers: SE buffer 202 (suitable for 3.3V LVTTL, 3.3V/2.5V LVCMOS, and 3.3V SSTL3 signaling), SE buffer 204 (suitable for 1.8V LVCMOS signaling), and SE buffer 206 (suitable for 2.5V SSTL2 and 1.5V/1.8V HSTL signaling). Similarly, SE input buffer 110 of FIG. 1 is actually implemented using three different input buffers: SE buffer 208 (suitable for 3.3V LVTTL, 3.3V/2.5V LVCMOS, and 3.3V SSTL3 signaling), SE buffer 210 (suitable for 1.8V LVCMOS signaling), and SE buffer 212 (suitable for 2.5V SSTL2 and 1.5V/1.8V HSTL signaling). Note that differential input buffer 106 is suitable for 2.5V/3.3V LVDS and 3.3V LVPECL signaling.

Furthermore, (2×1) muxes 112 and 114 of FIG. 1 are actually implemented using (4×1) muxes 214 and 216, respectively, where each (4×1) mux is controlled via a configurable 2-bit control signal.

Power (VCC) for input buffers 106 and 202-212 is applied at input pin 218. All of the input signaling standards of Table I can be supported by a VCC level of 3V.

Input termination level VTTA for input pin 102 is applied via input pin 220, programmable resistor RT1, and switch F1. If SE input clock signal REFA conforms to an LVTTL or LVCMOS standard, then switch F1 should be open. If SE input clock signal. REFA conforms to an HSTL or SSTL standard, then switch F1 should be closed.

Similarly, input termination level VTTB for input pin 104 is applied via input pin 222, programmable resistor RT2, and switch F2. If SE input clock signal REFB conforms to an LVTTL or LVCMOS standard, then switch F2 should be open. If SE input clock signal REFB conforms to an HSTL or SSTL standard, then switch F2 should be closed.

If differential input clock signal REFP/REFN conforms to an LVDS or LVPECL standard, then (i) switches F1 and F2 should both be closed, (ii) resistors R1 and R2 should both be set. to 50 ohms, and (iii) input pins 220 and 222 should be connected together, either shorted or via some appropriate external impedance.

Feedback Circuitry

FIG. 2 also shows a more detailed block diagram of feedback buffer circuitry 148 of FIG. 1. As shown in FIG. 2, feedback buffer circuitry 148 is actually implemented using a (4×1) mux 224 and four input buffers: differential buffer 226 (suitable for 2.5V/3.3V LVDS and 3.3V LVPECL signaling), SE buffer 228 (suitable for 2.5V SSTL2 and 1.5V/1.8V HSTL signaling), SE buffer 230 (suitable for 1.8V LVCMOS signaling), and SE buffer 232 (suitable for 3.3V LVTTL, 3.3V/2.5V LVCMOS, and 3.3V SSTL3 signaling), each of whose outputs is applied to a different input of mux 224, which is controlled by a configurable 2-bit control signal. The output of mux 224 of FIG. 2 is equivalent to the output of feedback buffer circuitry 148 of FIG. 1, and is appropriately shown being applied to the feedback port of PLL core 120 as feedback clock signal 146.

Power signal VCC applied at input pin 218 is also used to power input buffers 226-232. Feedback termination level VTTFBK for input pin 150 is applied via input pin 234, programmable resistor RTFBK, and switch FFBK. If SE feedback clock signal FBK conforms to an LVTTL or LVCMOS standard, then switch FFBK should be open. If SE feedback clock signal FBK conforms to an HSTL or SSTL standard, then switch FFBK should be closed.

As shown in FIG. 2, internal reference voltage level Int_Ref, which is generated internal to CD chip 100 based on VCC, is used to simulate differential signaling at differential buffer 226. This simulated differential signaling is preferably employed when the PLL reference clock signal output from mux 116 is based on a differential input clock signal REFP/REFN, so that the PLL feedback clock signal output from mux 224 will be based on a simulated differential clock signal derived from the single-ended output clock signals BANK_i[A/B] generated by CD chip 100 applied to pin 150.

The feedback buffer circuitry shown in FIG. 2 supports feedback clock signals 146 having the same relatively wide variety of different signaling standards shown in Table II for the output clock signals. This means than any one of output clock signals BANK_i[A/13] can be externally connected for application as feedback clock signal FBK at input pin 150.

Different Combinations of Signaling Standards

CD chip 100 can be configured to receive a relatively wide variety of different input clock signals and to generate a relatively wide variety of different output clock signals. In particular, CD chip 100 can be configured such that (1) SE input clock signals REFA and REFB conform to any combination of the SE signaling standards shown in Table I or (2) differential input clock signal REFP/REFN conforms to any of the differential signaling standards shown in Table I.

At the same time, and independent of the signaling standards of the input clock signals, the output clock signals BANK_i[A/B] generated by CD chip 100 can correspond to any combination of the SE signaling standards shown in Table II, with the sole constraint being that the two output clock signals for each bank (BANK_i[A] and BANK_i[B]) share the same VCCO and GNDO levels. Thus, for example, if VCCO for BANK_0 is 3.3V, then the two corresponding output clock signals (BANK_0[A] and BANK_0[B]) can conform to any combination of the 3.3V LVTTL, 3.3V LVCMOS, and 3.3V SSTL signaling standards, including both output clock signals conforming to the same signaling standard.

The feedback clock signal FBK applied to input pin 150 can be selected to be any one of the output clock signals BANK_i[A/B] independent of the signaling standard of the signals applied to input pins 102 and 104. In practice, however, when CD chip 100 is configured to receive SE input clock signals REFA and REFB, in order to reduce static phase offset, the output clock signal selected for feedback clock signal FBK typically conforms to the same signaling standard as one of input clock signals REFA and REFB. Moreover, in order for the PLL to operate properly, the output clock signal selected for feedback clock signal FBK should be a zero-delay output clock signal (if any) derived from the PLL's VCO clock signal.

Zero-Delay and Non-Zero-Delay Output Clocks

In addition to supporting different signaling standards, including simultaneous support of two or more different signaling standards, CD chip 100 of FIG. 1 can simultaneously generate zero, one, or more zero-delay (ZD) output clock signals and zero, one, or more non-zero-delay (NZD) output clock signals.

Each ZD output clock signal is based on the PLL's VCO clock signal 152, which itself is based on reference clock signal 118 from mux 116, which in turn may be based on any of differential input clock signal REFP/REFN, SE input clock signal REFA, or SE input clock signal REFB.

Similarly, each NZD output clock signal is based on clock signals that bypass the PLL. For example, one or more NZD output clock signals may be based on reference clock signal 118 bypassing the PLL via mux 144. At the same time, one or more other NZD output clock signals may be based on clock signal 124, while one or more further NZD output clock signals may be based on clock signal 126. Note that each of clock signals 124 and 126 may be based on either input clock signal REFP/REFA or input clock signal REFN/REFB.

Note further that, if mux 144 is configured to select divided PLL clock signal 154, then any NZD output clock signals can be based only on clock signals 124 and 126.

Alternatives

Although the present invention has been described in the context of CD chip 100 of FIGS. 1 and 2, the present invention is not limited to this particular embodiment. In general, clock distribution chips of the present invention may:

-   -   Support combinations of signaling standards other than those         listed in Tables I and II;     -   Include different numbers of divider circuits other than the         three divider circuits 130, 132, and 134;     -   Include switch fabrics different from (5×20) non-blocking switch         fabric 128, having different numbers of input ports and/or         different numbers of output ports, including blocking as well as         non-blocking switch fabrics;     -   Support numbers of different skew levels other than 16 and/or         different skew step sizes other than those based on VCO/8 and         VCO/4; and     -   Support other than two SE input clock signals and one         differential input clock signal. For example, a CD chip of the         present invention could have four input clock pins to support up         to four SE input clock signals and up to two differential input         clock signals, including simultaneously receiving one         differential clock signal at two of the input clock pins and one         or two SE clock signals at the other two input clock pins. As         another example, a CD chip of the present invention could have         only one input clock pin to support only a single SE input clock         signal and no differential input clock signals.

Unless explicitly stated otherwise, each numerical value and range should be interpreted as being approximate as if the word “about” or “approximately” preceded the value of the value or range.

It will be further understood that various changes in the details, materials, and arrangements of the parts which have been described and illustrated in order to explain the nature of this invention may be made by those skilled in the art without departing from the scope of the invention as expressed in the following claims.

The use of figure numbers and/or figure reference labels in the claims is intended to identify one or more possible embodiments of the claimed subject matter in order to facilitate the interpretation of the claims. Such use is not to be construed as necessarily limiting the scope of those claims to the embodiments shown in the corresponding figures.

It should be understood that the steps of the exemplary methods set forth herein are not necessarily required to be performed in the order described, and the order of the steps of such methods should be understood to be merely exemplary. Likewise, additional steps may be included in such methods, and certain steps may be omitted or combined, in methods consistent with various embodiments of the present invention.

Although the elements in the following method claims, if any, are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those elements, those elements are not necessarily intended to be limited to being implemented in that particular sequence.

Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments necessarily mutually exclusive of other embodiments. The same applies to the term “implementation.” 

1. A clock distribution chip comprising: a first clock input adapted to receive a first single-ended input clock signal; a second clock input adapted to receive a second single-ended input clock signal; input buffer circuitry coupled to the first and second clock inputs and adapted to select an input clock signal among the first single-ended input clock signal, the second single-ended input clock signal, and a differential input clock signal derived from the first and second single-ended input clock signals; a phase-locked loop (PLL) for receiving an input clock signal selected by the input buffer circuitry, the PLL adapted to generate a PLL clock signal based on the selected input clock signal; and at least one clock output for providing an output clock signal based on the PLL clock signal.
 2. The clock distribution chip of claim 1, wherein the input buffer circuitry includes: a differential buffer coupled to the first and second clock inputs; at least one first-input buffer coupled to the first clock input; at least one second-input buffer coupled to the second clock input; a first mux coupled to outputs of the differential buffer and the first-input buffer; a second mux coupled to outputs of the differential buffer and the second-input buffer; and a third mux coupled to outputs of first and second muxes, wherein the PLL is coupled to an output of the third mux to receive the selected input clock signal.
 3. The clock distribution chip of claim 2, wherein the at least one first-input buffer includes a plurality of first-input buffers, each first-input buffer adapted to handle a different signaling standard; the at least one second-input buffer includes a plurality of second-input buffers, each second-input buffer adapted to handle a different signaling standard; the first mux is coupled to the outputs of the first-input buffers and the differential buffer; and the second mux is coupled to the outputs of the second-input buffers and the differential buffer.
 4. The clock distribution chip of claim 1, wherein the clock output is programmable to support two or more of the following signaling standards: LVTTL, LVCMOS, HSTL, and SSTL.
 5. The clock distribution chip of claim 1 including: a feedback input adapted to receive an output clock signal from the clock output; and programmable feedback buffer circuitry coupled between the feedback input and the PLL and adapted to derive a feedback clock signal from the output clock signal received at the feedback input and to provide the feedback clock signal to the PLL.
 6. The clock distribution chip of claim 5, wherein the feedback buffer circuitry includes: a plurality of feedback buffers for different signaling standards, each adapted to receive the output clock signal from the feedback input and to provide a feedback clock signal; and a feedback multiplexer (mux) coupled to the outputs of the feedback buffers, the feedback multiplexer configurable to select a feedback clock signal from feedback buffers for the PLL.
 7. The clock distribution chip of claim 1 including: a plurality of clock outputs; and a programmable switch fabric coupled to the PLL and the input buffer circuitry to receive the first and second single-ended input clock signals and the PLL clock signal and to provide such clock signals to the plurality of clock outputs.
 8. The clock distribution chip of claim 7, wherein the programmable switch fabric is configurable to provide each of the first and second single-ended input clock signals and the PLL clock signal to any one or more of the clock outputs in a non-blocking manner.
 9. A clock distribution chip comprising: a first clock input adapted to receive a first single-ended input clock signal; a second clock input adapted to receive a second single-ended input clock signal; and input buffer circuitry coupled to the first and second clock inputs and adapted to select an input clock signal among the first single-ended input clock signal, the second single-ended input clock signal, and a differential input clock signal derived from the first and second single-ended input clock signals; a feedback input adapted to receive a clock signal; programmable feedback buffer circuitry adapted to derive a feedback clock signal from the clock signal received at the feedback input; and a phase-locked loop (PLL) coupled to the input buffer circuitry and the feedback buffer circuitry for generating a PLL clock signal based on an input clock signal selected by the input buffer circuitry and the feedback clock signal.
 10. The clock distribution chip of claim 9, wherein the input buffer circuitry includes: a differential buffer coupled to the first and second clock inputs; at least one first-input buffer coupled to the first clock input; at least one second-input buffer coupled to the second clock input; a first mux coupled to outputs of the differential buffer and the first-input buffer; a second mux coupled to outputs of the differential buffer and the second-input buffer; and a third mux coupled to outputs of first and second muxes, wherein the PLL is coupled to an output of the third mux to receive the selected input clock signal.
 11. The clock distribution chip of claim 10, wherein the at least one first-input buffer includes a plurality of first-input buffers, each first-input buffer adapted to handle a different signaling standard; the at least one second-input buffer includes a plurality of second-input buffers, each second-input buffer adapted to handle a different signaling standard; the first mux is coupled to the outputs of the first-input buffers and the differential buffer; and the second mux is coupled to the outputs of the second-input buffers and the differential buffer.
 12. The clock distribution chip of claim 9, wherein the feedback buffer circuitry includes: a plurality of feedback buffers for different signaling standards, each adapted to receive the clock signal from the feedback input and to provide a feedback clock signal; and a feedback multiplexer (mux) coupled to the outputs of the feedback buffers, the feedback multiplexer configurable to select a feedback clock signal from feedback buffers for the PLL. 